The present invention relates to a hydraulic motor for driving a shift transmission.
Together with a pump, usually a variable displacement pump, a typical hydraulic motor forms a so-called hydrostatic drive. The variable displacement pump and the hydraulic motor are connected to one another via corresponding lines and thus form a hydraulic force transmission system. This principle is based on the fact that the hydraulic motor receives the oil-volume stream produced by the variable displacement pump and converts it into a corresponding rotary movement. The oil discharged by the hydraulic motor is then guided back to the variable displacement pump at a lower pressure level. The variable displacement pump is usually equipped with a servo-adjustment means, with the result that the delivery quantity of the oil-volume stream can be adjusted in a stepless manner.
The conventional hydraulic motor can be used for driving a transmission which may be configured such that it can be shifted in one or more stages. The rotational speed of the hydraulic motor is usually reduced by the transmission.
Where a shift transmission is used, it is imperative that the hydraulic motor can be rotated to a slight extent as easily as possible during the shifting operation. It is then possible, within the shift transmission, for the forces acting on the tooth flanks and the shift forks during the shifting operation to be kept to a relatively low level, which, in addition to facilitating the shifting operation, minimizes the wear on the transmission. In particular, for driving a shift transmission, the latter can merely be shifted when the hydraulic motor is at a standstill, i.e., in a neutral position of the variable displacement pump. It is therefore desirable to overcome considerable friction torque within the hydraulic motor.
This friction torque is formed between a valve plate and a block cylinder of the hydraulic motor, which butt mechanically against one another when at a standstill. In this case, on the one hand, the charging pressure is acting on the surface between high-pressure piston and block-cylinder nodule. On the other hand, the spring force of a block-cylinder spring, which is arranged in the interior of the block cylinder, between the latter and the output shaft, cause the block cylinder to exert a contact-pressure force on the valve plate. Together with the material-induced friction of the abutment surface, this contact-pressure force forms a corresponding frictional force.
In order for it then to be possible to rotate the hydraulic motor for shifting purposes when at a standstill, it is imperative to overcome this frictional force. The torque which has to be initiated is generally referred to as a breakaway torque when at a standstill. Of course, a high breakaway torque for shifting a shift transmission proves to be extremely disadvantageous.
It is therefore a principal object of the present invention to keep the motor breakaway torque for shifting a shift transmission as low as possible, with the result that a small rotary movement is sufficient for the necessary shifting operation.
These and other objects will be apparent to those skilled in the art.
The device according to the invention within the hydraulic motor allows a considerable reduction in the contact-pressure force between the block cylinder and the valve plate of the hydraulic motor, with the result that, in this context, it is also the case that the friction torque between these components which has to be overcome is considerably reduced.
When the hydraulic motor is at a standstill, a hydraulic oil is introduced into the interior of the block cylinder via an additional valve device. In one embodiment of the invention, the hydraulic oil fed for this purpose is removed from the already present charging circuit of the hydraulic oil system of the hydraulic motor and the volume stream thereof is appropriately controlled via the valve device.
In this way, a hydraulic pressure field is built up in a defined surface between the block cylinder and the valve plate, this hydraulic pressure field counteracting the abovementioned contact-pressure force. The pressure thus building up in the interior of the block cylinder is capable of raising the block cylinder counter to the force of the central blockcylinder spring, on the one hand, and counter to the force of the charging pressure on the other hand, with the result that the mechanical connection between the valve plate and the block cylinder is eliminated. Thus, the breakaway torque of the intermediate hydraulic-oil layer produced in this way is considerably lower than in the case of direct mechanical support of the block cylinder on the valve plate.
In a preferred embodiment of the invention, the surface which is to be subjected to the action of the hydraulic pressure field is defined in that it is sunken in that end side of the block cylinder which is directed toward the valve plate, an intermediate gap being formed in the process. It is possible for a relatively large oil volume to be fed into this intermediate gap and thus for a considerably larger hydraulic pressure field, which can counteract the contact-pressure force, to be built up.
When the hydraulic motor is at a standstill, the supply of the hydraulic oil is maintained via a valve device. Following the shifting operation, the valve device opens again, with the result that the pressure field is dissipated and the hydraulic oil fed is discharged in a pressureless manner via the valve control means.
In a further configuration of the invention, the valve device is activated electronically by a pressure sensor which is arranged in the region of the output shaft. If the shaft is not rotating, for the purpose of shifting the transmission, the sensor transmits a corresponding shifting signal to the valve device. The invention makes it possible to reduce to a considerable extent the frictional force and/or the friction torque for the breakaway of the motor shaft, this reduction being approximately by a factor of five in comparison with actuation in the case of a hydraulic pressure field not being present. A non-synchronized transmission can thus be shifted without difficulty, as a result of which it is also possible to reduce to a considerable extent the mechanical outlay and the signs of wear, which are associated directly therewith, in the shift transmission.